This invention relates to a sensor apparatus and method and in particular to a sensor apparatus and method for use in seismic surveys.
To conduct a geophysical survey of an underwater area in oil exploration or prospecting, it is known to use a multi-component sensor in conjunction with returning echoes produced by a seismic source. Such a sensor comprises an orthogonal array of three geophones. The seismic source is normally impulsive, produced by explosives or airguns; or continuous, produced by vibrator trucks. The source produces sudden pulses of short duration which are reflected and detected by the geophones.
In practice, a number of sensors are inserted in a cable arrangement. In Ocean Bottom Cable or in shallow water operations this cable is fed from a seagoing vessel and allowed to settle on the seabed.
Each geophone is an electromechanical device sensitive to vibrations along its axis. The signals from each geophone are amplified, fed into data processing equipment and arranged to produce a seismic reflection record. Measurement and analysis of seismic vibrations in each axis gives information regarding underlying strata composition.
For the seismic data to be of use, it is necessary to know the orientation of the geophones. Thus, the geophones are mounted on gimbals. The gimbals allow the geophones to rotate under gravity to align with the vertical. Once the cable settles, one of the three geophones is aligned in the vertical axis, and the others are aligned in the horizontal plane, one along the line of the sensor and one at right angles to the line of the sensor. That is, the sensor is mechanically gimballed.
Since on reaching the seabed a sensor can settle in any direction, alignment of the horizontal geophones relative to North is calculated from the seismic data at the time of processing the full seismic data set.
The quality of results provided by such a mechanically gimballed sensor is limited by the fact that vibrations act on each geophone via the coupling arm of the gimbals. Hence whilst any vibration does indeed cause a corresponding movement on the respective geophone, this vibration passes to the geophone via the coupling arm of the gimbals.
The effect of this indirect experience of vibrations on the geophones is frequency dependent. The effect ranges from attenuation of the signal at some frequencies, to oscillation of the geophones at the resonant frequency of the coupling arm.
In addition, vibrations in the horizontal axes can produce a swing of the gimbals which translates the movement of the vertical sensor to accelerations in its sensitive axis. The sensor cannot differentiate between actual seismic vibrations, and those caused by this cross-coupling.
This conventional apparatus and technique is costly, which restricts its use. In addition, vibration acting on each geophone via the coupling arm of the gimbals, and cross-coupling through the gimbals, give a high probability of error.
An object of the present invention is to provide a means to simplify this apparatus and method, and thus to reduce the cost and increase the accuracy of the survey by a significant factor.